1. Field of the Invention
This invention relates to a diversity reception device attached to radio transmission equipment, specifically to a diversity reception device that weights and combines a plurality of reception signals.
2. Description of the Related Art
Currently, digital transmission equipment transmits the carrier signal modulated by the digital data signal (baseband signal) for effective data transmission. In modulating the carrier signal, several methods have been adopted such as: Amplitude Shift Keying (ASK) for varying the amplitude of the carrier signal according to the digital baseband signal (modulation signal); Frequency Shift Keying (FSK) for displacing the frequency of the carrier according to the modulation signal; Phase Shift Keying (PSK) for varying the phase of the carrier according to the modulation signal; and Quadrature Amplitude Modulation (QAM) for varying the amplitude and phase of the carrier independently according to the modulation signal.
It is well known that these digital modulation methods applied to mobile radio communication are affected by fading, a phenomenon caused by reflection or scattering of the electromagnetic wave, which seriously degrades the quality of the received signal. An effective method for supplementing reception level deterioration is the diversity reception which receives the signal over a plurality of lines.
Diversity reception is divided into the following types: Selection Combiner (SC) for selecting a signal with the highest reception level among the signals received in the plurality of lines before decoding; Equal-Gain Combiner (EGC) for combining all the received signals with equal level before decoding; and Maximal-Ratio Combiner (MRC) for weighting the received signals in proportion to each reception level and combining the signals before decoding.
Among the above three types, MRC most effectively combines the signals because the higher the reception level of the signal is, or the less noise-affected the signal is, the more weighted the signal is.
A conventional MRC is explained below.
FIG. 1 shows a block diagram of conventional diversity reception equipment with MRC with four lines for receiving signals. In the device, input terminals 101 to 104 receive the signals and phase shifters 105 to 108 equally arrange the phases of the carriers. Adder 109 then combines the signals and decoder 110 decodes the combined signal. In the above process, adder 109 combines the signals in linear form because each signal has been amplified in linear form.
FIG. 2 shows an Inphase Quadrature plane figure representing how current transmission equipment combines signals. For the sake of convenience it shows the case of combining two lines of signals.
S1 and S2 represent reception signals. S1S and S1N respectively represent a signal element and a noise element of S1. S2S and S2N respectively represent a signal element and a noise element of S2.
Generally, the level of each noise element is equal despite the size of received signal and the difference of receiving lines (hereinafter "branches"). That is why the reception signal in each branch is drawn as a point on the circumference with the same radius from signal elements S1S and S2S respectively (as .vertline.S1N.vertline.=.vertline.S2N.vertline.). In the diversity reception device with MRC in FIG. 1, signals received from each branch are combined in linear form, that is, S1 and S2 are combined as vectors, and the combined signal is input in the decoder.
However, since MRC requires a very high accuracy in synchronizing the carrier phases when combining the signals, convention reception devices with MRC tends to use an expensive Digital Signal Processor (DSP) to provide A/D conversion of the received signal for digital processing. Also, for combining the signal in linear form, an A/D converter with a wide dynamic range is required. In short, a reception device with MRC has problems in its size and cost.
Although MRC may be a most preferable combiner under propagation conditions when there are only random disturbances, like thermal noise, since it merely combines the received signals in linear form, MRC has a defect in that it receives interference waves such as delay waves, without processing. In particular, when a large incidence of such waves is included in the signal received in a branch with high reception level, the signal is heavily weighted despite its bad quality, decreasing the receiving performance.